Radio frequency identification (RFID) systems offer the possibility of inexpensive and efficient management of goods, with applications in inventory control and retail checkout, for example. Each item in an inventory of items, for example in a retail store or a warehouse, may be equipped with an RFID tag, that is, a tag containing an RFID device. The RFID device emits a signal that supplies information about the product to an RFID reader. The information may include product identification, such as manufacturer and item identifiers, such as those provided by a standard universal product code (UPC) bar code. The information may also include a unique serial number, so that each individual item may be easily identified by a reader. Typical RFID readers broadcast a radiofrequency (RF) signal. Tags within the range of the reader return a signal to the reader, frequently in the form of modulated backscatter of the signal emitted by the reader. RFID tags are preferably small and inexpensive. A small size allows for a tag to be placed on a small item. A low cost allows a tag to be placed on each item in an inventory at a reasonable overall cost. Producing small, low cost tags imposes constraints on the design of the tags. One effect of these constraints is that conventional RFID tags, particularly passive tags, tend to have relatively limited sensitivity. That is to say, conventional RFID tags must receive a relatively strong signal from the reader in order to receive power from and communicate with the reader. A conventional tag's need for a relatively strong signal limits the distance from an RFID reader at which the tag can operate. Many typical prior art passive tags can respond to a typical RFID reader from a range of approximately three feet.
In addition to having a relatively limited sensitivity even under ideal conditions, a typical prior art passive tag is sensitive to conditions that interfere with the reception of RF energy by the tag. For example, if a tag is placed on or near a product with high water content, such as on a soft drink container or a gallon of milk, the water in the product tends to absorb a significant portion of any RF energy reaching the product. Relatively little RF energy is available to power and communicate with the tag. Similarly, if an RFID tag is placed on metal container packaging a product, much of the RF energy coming from an RFID reader will be reflected away from the tag by the metal container, reducing the amount of RF energy available to the tag.
In many applications, it is desirable that every RFID tag be within range of at least one RFID reader deployed in a location. For example, the stock of a retail store may be arranged on shelves throughout the store, with a number of RFID readers deployed throughout the store. Conventional tags must typically be placed within relatively short distances from an RFID reader, requiring that a sufficiently large number of readers be deployed so that each tag is a relatively short distance from at least one reader.
Deploying a large number of readers leads to significant expense, for example the cost of the readers and the cost of wiring for the readers. In addition, the use of a large number of wired readers requires rerouting of a large amount of wiring for the readers whenever the location is remodeled. Further, the use of a large number of readers increases the probability that at least one reader will fail and therefore increases the cost of maintaining the reader system due to the frequent need for repair and replacement of readers.
There exists, therefore, a need for an RFID tag having a relatively high sensitivity to RF energy emitted by an RFID reader.